1. Field of the Invention
This invention relates to a switch opening/closing detection apparatus wherein opening/closing of a switch having first and second contacts is detected by a control unit connected to the switch through a wiring line. More particularly, to a switch opening/closing detection apparatus suitable to detect opening/closing of a neutral detection hydraulic switch for detecting connection/non-connection of a clutch of a motorcycle.
2. Description of Background Art
As shown in FIG. 4, a switch opening/closing detection apparatus is known in the prior art wherein, in order to detect opening/closing of a switch 2 grounded at one of contacts thereof, the other contact of the switch 2 and a pulled up port 3 of an ECU (electronic control unit) 4 are connected to each other by a wiring line (harness) 5.
When the switch 2 is closed, the potential of the port 3 becomes the low level, but when the switch 2 is opened, the potential of the port 3 becomes the high level. Therefore, opening/closing of the switch 2 can be detected by a CPU 6.
This switch opening/closing detection apparatus detects in error that, when the wiring line 5 is broken, the switch 2 is in an open state irrespective of whether the switch 2 is open or closed.
Regarding a liquid pressure switch grounded at a contact thereof and having a normally closed contact which is open when a liquid pressure is applied thereto but is closed when no liquid pressure is applied, a technique which solves the detection error described above has been proposed. See, for example, Official Gazette of Japanese Patent Laid-Open No. Hei 8-2404 (FIG. 1).
As shown in FIG. 5, one of contacts is divided into two contacts, and one of the two contacts is connected to a port 7 of an ECU 9 which has a pull-down resistor built therein through a wiring line 5 while the other of the two contacts is connected to a power supply Vcc through an alarm indicator 11.
Where such a connection scheme as described above is used, when liquid pressure is applied to the switch 2 and the switch 2 is open, if the wiring line 5 is broken, then the alarm indicator 11 is turned off, and consequently, the breakage of the wiring line 5 can be detected. In other words, failure detection of an apparatus which incorporates the switch 2 can be carried out.
It is to be noted that, as a hydraulic switch having a normally open contact which is open when no hydraulic pressure is applied, a neutral detection hydraulic switch for detecting connection/non-connection of a clutch of a motorcycle is provided on an oil passage of a clutch. See, for example, Official Gazette of Japanese Patent Laid-Open No. 2007-290580 (FIG. 12, reference numeral 107).
However, in the technique disclosed in Official Gazette of Japanese Patent Laid-Open No. Hei 8-2404 (FIG. 1) above, even if the switch 2 normally operates between open and closed states, if the wiring line 5 is broken, then the apparatus in which the switch 2 is incorporated decides a failure. Therefore, there is a problem that the apparatus in which the switch 2 is incorporated is disabled.